Wireless devices are usually powered by batteries. Since batteries have a finite life, they need to be replaced from time to time. But replacing batteries in wireless devices that are hard to access can be difficult and costly. Furthermore, batteries can fail at inopportune, unpredictable times. In applications where temporary outages of remote devices are not acceptable, batteries must be replaced preventively on a regular basis. The problem is especially troublesome for remote RF receivers because they must be on continuously in order to be able to receive a transmission. When the remote device includes a transmitter, the need to preserve battery life can often constrain the way in which the device is used. For example, the interval between transmissions may be lengthened, and transmission power may be lowered. Although this can prolong battery life, it can reduce the utility of the remote wireless device.